Electrical interconnections between transistors and other semiconductor circuit components are typically formed using "metal" interconnection lines. While doped polycrystalline silicon can be used for some circuit interconnections, virtually all semiconductor circuits use at least one layer of metal interconnection lines. Such metal interconnection lines are typically formed by depositing a thin film of aluminum or aluminum-copper alloy on the wafer, masking the aluminum layer with resist to define a set of metal interconnection lines, and then anisotropically etching the portions of the metal layer not covered by resist. Reactive ion etching (plasma etching) of metal thin films is usually performed using a plasma in which the wafer is bombarded with ions that react with and remove the exposed regions of metal. Plasma etching is performed in a vacuum chamber, and the etcher's vacuum pumping system removes most, but not all, of the reaction products. Advantages of plasma ion etching over conventional wet etching processes include the possibilities of process automation, less undercutting of wall profiles, and higher packing density.
Halogens, such as chlorine ions, are almost always used when plasma etching aluminum and aluminum alloy films on semiconductor wafers. It is well known among semiconductor manufacturers that the reactive ion etching process creates halogen compounds, which will hereinafter assumed, for the purposes explanation, to be chlorine compounds. It is also well known that some of the chlorine compounds produced by the plasma etching process remain on the wafer as a residue after completion of the etching process, and thereafter hydrolyze by reaction with ambient moisture to produce hydrochloric acid (HCl). The hydrochloric acid resulting from this reaction is able to dissolve aluminum from the sidewalls of the etched conductors where the passive native oxide has not regrown sufficiently after the etching process. The problem is compounded by the effect that this reaction scheme is self-perpetuating in the presence of moisture; as long as moisture is available, corrosion of the aluminum will continue. The extent and rate of the metal corrosion reactions depend on many factors, including the amount of chlorine-containing compounds remaining on the wafer after the metal etching step. However, it is believed that even a small amount of chlorine can cause significant corrosion damage and semiconductor device failures.
The corrosion resulting from exposure of aluminum interconnects to air frequently leads to rapid and catastrophic corrosion of the etched conductors. Such corrosion induces failure of integrated circuits and reduces the yield of fabrication processes. As a result, almost all aluminum etch processes include post dry-etch corrosion prevention cycles. These cycles are intended to delay the onset of the corrosion mechanism until a later time in the manufacturing process when a thorough passivation process can be achieved.
Most techniques for preventing corrosion attempt to remove chlorine from the wafer. For example, heating the wafer causes some of the volatile chlorine-containing compounds to evaporate from the wafer surface. One technique for heating the wafer is to irradiate it with an infrared lamp while under vacuum. Another method used to remove chlorine from the wafer surface is to rinse the wafer in deionized water or other solutions before or while stripping the resist mask layer on top of the metal connection lines.
The corrosion of structures formed by reactive ion etching has also been attempted to be suppressed through formation of a protective layer on the etched surface. For example, thin polymers have been deposited on the etched wafer surface to inhibit attack of the patterned metal. This is typically accomplished by immersing the wafers in a hydrogen-containing fluorocarbon plasma, using CHF.sub.3. Similarly, it has been found that oxidizing the aluminum surface by heating the wafer in an oxygen ambient in a furnace helps prevent corrosion by forming an oxide barrier that substantially prevents the corrosion reaction. The layer of resist on top of the metal is generally removed before the oxidation step, because the temperature used during oxidation can make it difficult to later remove the resist. As a result, the wafer is exposed to the atmosphere prior to oxidation, and therefore corrosion may still occur between completion of the etching step and oxidation of the aluminum.
A factor making the above described corrosion problem even more critical is the growing use of metal interconnects in which the aluminum or aluminum-copper layer is "sandwiched" between upper and lower thin tungsten/titanium (W/Ti) layers. A description of such metal interconnect layers and a process for making them can be found in U.S. Pat. No. 4,019,234. The W/Ti layer below the aluminum layer prevents diffusion of aluminum into the silicon substrate, and the W/Ti layer above the aluminum layer forms a compressive cap that prevents hillocking of the aluminum and also reduces reflectivity of the metal layer, thereby facilitating photolithographic masking of the metal interconnection layer.
It has been the inventor's experience that the extent and rate of metal corrosion is significantly worse for metal layers comprising a W/Ti-Aluminum-W/Ti metal sandwich than for ordinary aluminum metal layers. In particular, the amount of corrosion which occurs between metal etching and resist stripping appears to be more significant for such metal sandwiches, causing more device failures. While the mechanism that makes the corrosion problem worse for such wafers is not known, it can be speculated that the interface between the W/Ti layers and its neighbors creates a electrical potential or field that accelerates the rate of the corrosion process.
It is therefore a primary object of the present invention to provide a processing method which significantly and reliably reduces metal interconnect corrosion.